As products get smaller, there is greater demand for micro-electrical-mechanical systems (MEMS), micro-optical devices and photonic crystals. With this demand, there is an associated increased interest in micro- and nano-machining. There are many applications for MEMS. As a breakthrough technology, allowing unparalleled synergy between fields such as biology and microelectronics, new MEMS applications have emerged and many more may emerge in the near future, expanding beyond those currently known. Additional applications in quantum electric devices, micro-optical devices and photonic crystals are also emerging.
As an example, photonic crystals represent an artificial form of optical materials that may be used to create optical devices with unique properties. Photonic crystals have optical properties that are analogous to electrical properties of semiconductor crystals and, thus, may allow the development of optical circuitry similar to present electrical semiconductor circuitry. The feature sizes used to form photonic crystals and the precise alignment requirements of these features complicate manufacture of these materials. Improved alignment techniques and reduced minimum feature size capabilities for micromachining systems are still under development. One reason why optical circuits have not been widely implemented is because there are manufacturing problems related to making photonic devices meet index of refraction specifications.
As another example, methods are known for reducing the infrared emissions of an incandescent light source by using an optical microcavity. U.S. Pat. No. 5,955,839, entitled Incandescent Microcavity Light Source having Filament Spaced from Reflector at Node of Wave Emitted, describes microelectronic processing techniques to form a filament in a single optical microcavity. The presence of the optical microcavity provides greater control of the directionality of emissions and increases the emission efficiency in a given bandwidth (for example, the 1-2 micron near infrared band).
A similar type of efficiency gain may be obtained by forming an array of microcavity holes in an incandescent light source. Such a light source may, for example, have microcavities of between 0.5 micron and 10 micron in diameter. While features having these small dimensions may be formed in some materials using standard microelectronic processing techniques, it is difficult to form these features in metals such as tungsten, which is commonly used as an incandescent filament.
Laser light may be used to drill holes in, or otherwise machine a work piece containing glass or silicon or other dielectric material. The behavior of light in such a material may be better understood by analogy to the behavior of electricity in a conventional crystal. Crystals are characterized by a periodic arrangement of atoms or molecules. The lattice of atoms or molecules may introduce gaps in the energy band structure of the crystal through which electrons cannot propagate. A photonic crystal is a lattice of discontinuities in the refractive index of a material. One example is a lattice of holes in a waveguide. If the dielectric constants of the waveguide material and the material in the holes are sufficiently different, light is substantially confined by these interfaces. Scattering of the light at these interfaces can produce many of the same effects for photons as effects produced for electrons by the lattice of atoms or molecules.
Typically, ultrafast lasers in the visible (dye laser) or IR range (the fundamental wavelength of Ti: Sapphire or Nd:YLF) have been used for laser machining applications. It is known that the minimum spot size of a focused laser beam, having a Gaussian beam profile, is approximately 2.44 times the f# of the objective lens, times the peak wavelength of the laser, i.e. the spot size is proportional to the peak wavelength. Thus, in a system where a visible or an IR laser is used for nanomachining, the spot size is undesirably large for forming submicron features, even if high numerical aperture (low f#) optics are used. For example, if a Ti: Sapphire laser having an 800 nm peak wavelength and optics with an f# of 1 at 800 nm are used, the minimum size beam spot has a diameter of 1952 nm.
Even with this disadvantage, in late 1999 and early 2000, a frequency doubled Ti: Sapphire laser with a peak wavelength of 387 nm has been used to machine approximately 200 nm air holes in plain Si-on-SiO2 substrate. This submicron feature was achieved by controlling the fluence of a laser beam spot so that ablation only occurs near the intensity peak of the laser beam spot. This technique, however, has a number of drawbacks for precise nanomachining, since a center of the area actually machined may be somewhat offset from a center of the intensity profile. This uncertainty of the machining center may be induced by defects or imperfections of the material being processed, or may be due to slight pulse-to-pulse variations in the beam profile. In addition, as the feature sizes on the substrate decrease to less than or equal to the size of the wavelength of the beam, the image formed on the substrate is blurred.
Furthermore, it is difficult to accurately align a laser beam to produce multiple holes positioned in a desired lattice arrangement with an accuracy needed for an effective photonic structure. A current method of producing holes (single and multiple holes) uses a moveable work piece holder on which a photonic crystal is mounted. The laser beam is aligned at a desired location on the crystal by maintaining the laser beam in a single location and moving the work piece holder with the work piece mounted onto it. The holder, however, cannot be moved with a level of accuracy suitable for manufacturing photonic crystals.
What is needed is a better way to mass manufacture a photonic crystal including a way to drill submicron holes or cavities in a substrate where the feature size is less than or equal to the wavelength of the laser beam, and accomplish the drilling simultaneously using parallel beams of light. The present invention addresses such need. What is also needed is a better way of making an array of microcavities in an incandescent light source, such as a filament made from tungsten. The present invention also addresses this need.